As the strength of components such as steel sheets for automobiles or transport equipment has been growing, the processability such as ductility and stretch-flangeability has decreased making it difficult to form parts of complex shape by pressing. Accordingly, techniques ensuring excellent processability even for high-strength steel sheets need to be provided. Further, for example, steel sheets for automobiles are also required to excel in weldability because the parts are mainly assembled by spot welding. A cross tensile strength (CTS) (measured by the cross tension test) obtained by spot welding together the sheets of the same steel and performing a cross tension test in the peeling direction is generally used as quality parameter of welded portions of high-strength steel sheets.
The following patent documents suggest techniques for improving the processability, among the required characteristics, of high-strength steel sheets.
In Patent Literature 1, in particular, B is included, the ratio of amounts of Ti and N is adjusted, as appropriate, and a steel structure is made to include mainly the tempered martensite, with the retained austenite or additionally ferrite and martensite having the desired area ratio. As a result, both the strength and the moldability (elongation and stretch-flangeability) of the steel sheet can be improved. Among the structural components, it is indicated that by including the retained austenite at 5 area % or more, the total elongation (EL) is ensured. However, Patent Literature 1 is restricted to investigating strengthening and moldability, and weldability is not considered.
In Patent Literature 2, the strength of the martensite structure is increased without increasing the volume ratio of martensite, and the decrease in ferrite volume which ensures ductility is reduced to a minimum, and the volume ratio of ferrite is controlled to 50% or more. As a result, a high-strength cold-rolled steel sheet and a high-strength galvanized steel sheet are obtained in which ductility and delayed fracture resistance can be ensured and also a maximum tensile strength of 900 MPa or higher can be ensured. However, similarly to Patent Literature 1, weldability is not investigated.